Signal detecting frequency counter

ABSTRACT

The invention provides an apparatus and a method for determining the frequency of a radio frequency signal. The coherency of a received signal is determined and only the frequency of a coherent signal is determined. Zero amplitude transitions of coherent signals are counted in respective time periods. If the counts in each time period agree within a quantization error, the accumulated counts for a fixed time period are employed to determine a frequency. The frequency determining apparatus and method are particularly useful with a frequency agile radio receiver for detecting the presence of radio frequency signals from nearby mobile and stationary transmitters. Upon determination of the frequency of the nearby transmitter, the channel of transmission can be determined from a memory within the receiver and the receiver can be tuned to monitor the transmission. Additional tests to verify the frequency determination may be applied using squelch and window detector circuits of the radio receiver.

This application is a continuation of U.S. patent application Ser. No.09/860,506 filed May 21, 2001, now U.S. Pat. No. 7,006,797.

FIELD OF THE INVENTION

The invention concerns an apparatus for determining the frequency of areceived coherent radio frequency signal and a process for determiningthe frequency of a coherent radio frequency signal. The apparatus andprocess are particularly useful in radio equipment, such as frequencyagile radio receivers and transceivers. The apparatus and method mayidentify a channel including the frequency determined for automaticallytuning the radio receiver or transceiver to the channel including theradio frequency signal. By automatically tuning the receiver ortransceiver, a transmission can be monitored or a conversation inprogress can be joined.

BACKGROUND

Apparatus for determining the frequency of an electrical signal has longbeen known. A conventional apparatus for determining the frequency isusually referred to as a frequency counter and frequently includes avisual display showing in digital form the frequency measured. Theapparatus typically counts the number of cycles of an incoming signalduring a sampling period. Some of the prior art frequency countersinhibit display of a count unless the signal detection method used bythe counter indicates that a dominant signal is present in signals inputto the frequency counter.

Frequency agile radio receivers are available that allow a listener tomonitor conversations on each of numerous fixed frequencies. Thereceiver may monitor fixed frequencies or channels, tuning one at atime, typically in response to programmed instructions. These receiverstypically include a memory in which frequency information for tuning thereceiver is stored. The receivers are usually programmable for alteringor establishing the stored frequency information and monitoring process.The programming feature permits the stored frequency information to bechanged, for example, in response to location changes or changes infrequency allocation information. However, when a frequency agile radioreceiver is in operation in an area where there are many transienttransmitters, for example, mobile transceivers. The user of thefrequency agile radio receiver may not know the frequencies oftransmission of the nearby transmitters. The typical receiver isincapable of determining the frequencies of these transmissions so thereceiver cannot tune to monitor them. Since listeners desire to monitorthese nearby transmissions, it is desirable to include, within theradio, apparatus for determining the channel including the frequency ofthese nearby transmissions and, preferably, to provide for manual orautomatic tuning of the radio receiver to that channel for monitoringthem.

Some prior art frequency counters for tuning radio receivers have beenavailable in housings separate from the receivers and connected to thereceivers by cables, making use awkward. Counting of the frequency andtuning of the associated receiver are disadvantageously slow because thespecial purpose counters require at least two complete frequencydeterminations before producing an output.

Accordingly, there is a need for a frequency determining and radiotuning apparatus that can rapidly and accurately determine the frequencyof a received radio frequency signal, that can ensure that only thefrequency of a coherent signal is determined, and that, in applicationto a radio receiver, can tune the receiver to the channel including thefrequency determined before loss of the signal.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention to provide a frequencydetermining apparatus and method that rapidly determines the frequencyof a received radio frequency signal, measuring the frequency only ifthe signal is coherent.

It is another object of the invention to provide a frequency determiningapparatus and an interconnected frequency agile radio receiver and amethod for automatically tuning the radio receiver to a channelincluding a frequency determined by the frequency determining apparatus.

According to a further object of the invention, a frequency determiningapparatus and an interconnected frequency agile radio receiver determinethe frequency of a received radio frequency signal, determine whetherthe signal is from a mobile transmitter, and, is so, tune to receivesignals from a repeater transmitter associated with the mobiletransmitter.

Yet a further object of the invention is to provide a frequencydetermining apparatus and a frequency agile radio receiver in the samehousing and sharing common circuitry.

It is still another object of the invention to provide a radiotransceiver incorporating a frequency determining apparatus and methodso that a channel including the frequency of a received signal isaccurately and quickly determined and the transceiver transmitter andreceiver are automatically tuned to the frequency determined for joiningan ongoing conversation.

According to one aspect of the invention, a frequency determiningapparatus includes a counter for counting amplitude transitions of aradio frequency signal in each of a plurality of time periods ofidentical duration, a first register for storing counted zero amplitudetransitions for a selected one of the time periods, and a secondregister for storing an accumulated count of zero amplitude transitionscounted during a total counting time including the plurality of timeperiods; and comparing means for comparing counted zero amplitudetransitions for each time period other than the selected time periodwith the counted zero amplitude transitions for the selected timeperiod, the second register discarding the accumulated count if thecounted zero amplitude transitions for any of the time periods otherthan the selected time period are different from the counted zeroamplitude transitions for the selected time period by more than aquantization error, wherein the frequency of the radio frequency signalis indicated by the accumulated count stored in the second register whenthe total counting time ends.

According to a second aspect of the invention, the frequency of a radiosignal is determined by counting zero amplitude transitions of a radiofrequency signal in each of a plurality of time periods of identicalduration, storing counted zero amplitude transitions for a selected oneof the time periods storing an accumulated count of zero amplitudetransitions during a total counting time including the plurality of timeperiods; comparing counted zero amplitude transitions for each timeperiod other than the selected time period with the counted zeroamplitude transitions for the selected time period, discarding theaccumulated count if the counted zero amplitude transitions for any ofthe time periods other than the selected time period are different fromthe counted zero amplitude transitions for the selected time period bymore than a quantization error, accumulating counted zero amplitudetransitions until the total counting time is reached; and determiningthe frequency of the radio frequency signal from the accumulated countstored when the total counting time is reached.

According to a third aspect of the invention, a radio apparatuscomprises a radio receiver including an antenna for collectingelectromagnetic energy, and receiver circuits for processing radiofrequency signals received through the antenna and for producing anoutput, and a frequency determining apparatus including a counter forcounting zero amplitude transitions of a radio frequency signal in eachof a plurality of time periods of identical duration, a first registerfor storing counted zero amplitude transitions for a selected one of thetime periods a second register for storing an accumulated count of zeroamplitude transitions counted during a total counting time including theplurality of time periods; and comparing means for comparing countedzero amplitude transitions for each time period other than the selectedtime period with the counted zero amplitude transitions for the selectedtime period, the second register discarding the accumulated count if thecounted zero amplitude transitions for any of the time periods otherthan the selected time period are different from the counted zeroamplitude transitions for the selected time period by more than aquantization error, wherein the frequency of the radio frequency signalis indicated by the accumulated count stored in the second register whenthe total counting time ends and the frequency determining apparatussupplies the frequency determined to the radio receiver for tuning ofthe radio receiver.

According to a fourth aspect of the invention, a radio receiver is tunedto a channel including a frequency of a received radio frequency signal,the frequency of the received signal being determined by counting zeroamplitude transitions of a received radio frequency signal in each of aplurality of time periods of identical duration, storing counted zeroamplitude transitions for a selected one of the time periods storing anaccumulated count of zero amplitude transitions during a total countingtime including the plurality of time periods; comparing counted zeroamplitude transitions for each time period other than the selected timeperiod with the counted zero amplitude transitions for the selected timeperiod, discarding the accumulated count if the counted zero amplitudetransitions for any of the time periods other than the selected timeperiod are different from the counted zero amplitude transitions for theselected time period by more than a quantization error; accumulatedcounted zero amplitude transitions until the total counting time isreached; determining the frequency of the received radio frequencysignal from the accumulated count stored when the total counting time isreached; and tuning the radio receiver to a channel including thefrequency determined.

According to a fifth aspect of the invention, the frequency of areceived radio frequency signal is determined by counting zero amplitudetransitions of the radio frequency signal and storing an accumulatedcount of zero amplitude transitions counted; determining the frequencyof the received radio frequency signal from the accumulated count ofzero amplitude transitions accumulated during a total counting time;tuning a radio receiver having a pass band to a channel including thefrequency determined; and, after tuning, determining whether a signal isbeing received within the pass band of the radio receiver for confirmingaccuracy of the frequency determined.

According to a sixth aspect of the invention, a radio apparatuscomprises a radio receiver including an antenna for collectingelectromagnetic energy, and receiver circuits for processing radiofrequency signals received through the antenna and producing an outputand including a tuner tuning only to allocated frequency channels, and afrequency determining apparatus for determining the frequency of a radiofrequency signal received from the antenna, wherein the radio receiverincludes a memory coupled to the tuner and to the frequency determiningapparatus for storing frequency information for channels allocated tomobile transmitters and frequency offsets between mobile transmittersand repeater transmitters repeating transmissions of the mobiletransmitters and, when the radio receiver determines that a channel forwhich a radio frequency signal has been received and the frequencydetermined by the frequency determining apparatus is allocated to amobile transmitter, the tuner is tuned to a frequency offset from thechannel including the radio frequency signal for which the frequency hasbeen determined, by the offset stored in the memory, for detecting aradio frequency signal transmitted from a repeater transmitter.

According to a seventh aspect of the invention, a method of tuning aradio receiver comprises determining the frequency of a received radiofrequency signal at a radio receiver including a tuner tuning only toallocated frequency channels and a memory coupled to the tuner andstoring frequency information for channels allocated to mobiletransmitters and frequency offsets between mobile transmitters andrepeater transmitters repeating transmissions of the mobiletransmitters; and determining whether a channel including a frequencydetermined from a received radio frequency signal is allocated to amobile transmitter, and, if so, tuning the radio receiver to a frequencyoffset from the channel including the radio frequency signal for whichthe frequency has been determined, by the offset stored in the memory,for detecting a radio frequency signal transmitted from a repeatertransmitter.

According to an eighth aspect of the invention, a radio apparatuscomprises a frequency agile radio receiver including an antenna forcollecting electromagnetic energy, a bandpass filter connected to theantenna and having a pass band passing radio frequencies within a firstrange of radio frequencies, and rejecting frequencies outside the firstrange of frequencies; receiver circuits for processing radio frequencysignals received through the antenna and producing an output, a radiofrequency preamplifier amplifying a radio frequency signal received fromthe antenna through the bandpass filter; and a microprocessorcontrolling tuning of the frequency agile radio receiver; and afrequency determining apparatus for determining the frequency of a radiofrequency signal received from the antenna and through the bandpassfilter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a radio receiver apparatus incorporating anembodiment of the frequency determining apparatus and method accordingto the invention.

FIG. 2 is a flow chart illustrating the process by which the apparatusof FIG. 1 operates.

FIG. 3 is a flow chart illustrating a frequency determining methodaccording to an embodiment of the invention.

FIG. 4 is a block diagram of a radio transceiver apparatus incorporatingan embodiment of the frequency determining apparatus and methodaccording to the invention.

DETAILED DESCRIPTION Radio Receiver with Frequency Determination

FIG. 1 is a block diagram of a frequency agile radio apparatus includingan apparatus for determining the frequency of a radio frequency signalaccording to an embodiment of the invention. FIG. 2 is a flow chartillustrating a method of operation of radio receiver apparatus like thatof FIG. 1 according to an embodiment of the invention. FIG. 3 is a flowchart illustrating the frequency determination method and apparatusexemplified by FIGS. 1 and 2. In the following description, emphasis isplaced upon use of the invention in connection with a frequency agilereceiver that receives radio frequency signals. However, the inventionhas several aspects and is not limited to application in radioreceivers. For example, the frequency determination aspect of theinvention may be generally applied to any frequency determination of aradio frequency signal, for example, through a directly connectedcircuit, without the reception of electromagnetic waves propagated froma transmitting antenna.

In the embodiment of FIG. 1, a radio receiver A that is frequency agile,i.e., can be tuned to various frequencies by an internal microprocessor,is interconnected with a frequency determining apparatus B. The receiverA includes an antenna 1 connected to a bandpass filter 3. The bandpassfilter 3 desirably eliminates unwanted electromagnetic energy, includingnoise, outside a frequency range of interest. Further, although shown asa discrete element in FIG. 1, the bandpass filter 3 may be a part of aradio frequency amplifier or amplifiers used in conjunction with thefilter element rather than a distinct circuit. In the illustratedembodiment, the bandpass filter 3 includes multiple filter elements. Theillustrated embodiment includes five such filter elements. Eachindividual bandpass filter element has a particular pass band, differentin frequency range from the pass bands of the other filter elements. Inthe illustrated embodiment, bandpass filter elements 3-1, 3-2, 3-3, 3-4,and 3-5 are illustrated. As discussed below, only one of these filterelements is connected at any particular time for providing to the signalprocessing circuitry of the receiver A whatever radio frequency signalsare received at the antenna 1 and that fall within the pass band of thefilter element that is connected at that time.

In a frequency determination mode, each of the bandpass filter elementsis connected, in sequence, for a time sufficient for determining thefrequency of a received signal falling within the pass band of theconnected filter element. As a non-limiting example, when the inventionis applied to a radio receiver intended to monitor communicationsbetween transceivers, one or both of which may be mobile or fixed, thefilter element 3-1 may have a bandpass frequency range of 30-54 MHz. Thefrequency ranges of the other filter elements may be, for example,108-136 MHz, 136-174 MHz, 406-512 MHz, and 806-956 MHz. These frequencyranges, as an example, include standard communication frequency bandsand avoid commercial broadcast frequencies. By describing a bandpassfilter with five filter elements, it is not intended to require amultiple element filter in all or any applications of the invention. Asingle bandpass filter or even no bandpass filter may be appropriate forparticular applications of the invention, for example, in simplefrequency determinations.

The Frequency Determining Apparatus

The output signal from the bandpass filter 3 is, preferably, amplifiedin a radio frequency preamplifier 5 to improve the sensitivity of theapparatus to relatively weak radio frequency input signals. Theamplified radio frequency signal provided by the preamplifier 5 issupplied to the signal processing circuitry of the radio receiver A andto the frequency determining apparatus B. The frequency determiningapparatus B includes an optional radio frequency preamplifier 7. Thispreamplifier 7 is optional in the illustrated receiver because thepreamplifier 5 of the radio receiver may provide sufficient gain forboth the receiver A and the frequency determining apparatus B. In fact,a single preamplifier may serve both of the frequency determiningapparatus and the frequency agile radio receiver. The joint usage of asingle element is an example of one aspect of the invention in which acommonly housed frequency counter and radio receiver share many commoncircuits, providing substantial economies. When the frequencydetermining apparatus B stands alone and is not part of nor connected toa radio receiver, inclusion of the preamplifier 7 is highly desirable toincrease sensitivity and improve overall performance.

The further amplified radio frequency signal from the preamplifier 7 issupplied to an optional first prescaler 9. When the highest frequencysignal for which the frequency is to be determined is very high, it isuseful to divide the frequency of the signal, i.e., to lower thefrequency of the signal to be processed, before attempting to determinethe frequency of the signal. Otherwise, the processing circuitry fordetermining the frequency can become quite complicated and expensive,particularly in processing frequencies, for example, that approach oneGHz. The first prescaler 9 is a commercially available integratedcircuit, for example, the SA701 available from Phillips Semiconductors,Sunnyvale, Calif. Preferably, the first prescaler 9 has a constantdivisor by which it divides the frequency of the signal supplied to itsinput, regardless of the frequency, in order to produce a reducedfrequency signal at its output.

It is preferred, in the invention, that the frequency of the signaloutput by the first prescaler 9 not exceed about 10 MHz for ease ofprocessing the output signal from the first prescaler 9 without unusualcircuitry. For example, the divisor of the first prescaler 9 may be setto 128, with regard to the specific, non-limiting example of thefrequency ranges of the bandpass filter 3 previously described, toensure that the output frequency of the prescaler does not exceed 10MHz. In another, less preferred, embodiment, if the divisor of the firstprescaler 9 is variable, for the example for the frequency rangesprovided by the filter elements 3-1-3-5 described above, the divisor ofthe first prescaler 9 might be set at 8 for the frequency range of thefilter element 3-1, 16 for the filter element 3-2, 32 for the filterelement 3-3, 64 for the filter element 3-4, and 128 for the filterelement 3-5. A connection to the first prescaler 9 from the radioreceiver A is shown in FIG. 1 that is only present if the divisor of thefirst prescaler is variable and controlled in coordination with theselection of one of bandpass filter elements 3-1-3-5 by the radioreceiver.

As shown in FIG. 1, the first prescaler 9 supplies an output signal to amicrocontroller 11 of the frequency determining apparatus B. Since thepreamplifier 7 and the prescaler 9 are both optional, the frequencydetermining apparatus B may, in some embodiments, consist only of themicrocontroller 11. The microcontroller 11 may be, for example, a PIC12C672 microprocessor commercially available from Microchip Technologyof Chandler, Ariz. The microcontroller 11 receives the radio frequencysignal, adjusted by the preamplifier 7 and the first prescaler 9, ifpresent. The microcontroller 11 provides an output signal, in theembodiment of FIG. 1, to the radio receiver A.

The Radio Receiver

Turning again to the radio receiver A, the radio receiver A in theportions now discussed may be entirely conventional, except for theinteraction with the frequency determining apparatus B. For example, theradio signal processing and control circuits may be those of the UnidenModel BC245XLT, a frequency agile radio receiver tuned in receivingfrequency in response to execution of programmed instructions by aninternal microprocessor. The circuits in the radio receiver that areconventional are, therefore, only described with respect to their wellknown functions.

In the radio receiver A, the amplified radio frequency signal from thepreamplifier 5 is supplied to conventional receiver circuits 21. Theconventional receiver circuits 21 include, for example, a firstdetector, a local oscillator and associated tuner for producing anintermediate frequency (IF) signal, IF amplifier stages, and a seconddetector circuit. The output of these conventional receiver circuits 21is a demodulated signal including any received audio and a DC componentindicating the location of the received signal within the pass band ofthe receiver. The receiver pass band is different from the pass bands ofthe bandpass filter 3 and its filter elements 3-1 3-5 described above.The receiver pass band is typically relatively narrow, for example, 25kHz in width.

The signal output by the conventional receiver circuits 21 is suppliedto three circuits, namely, a window detector 23, a squelch detector 25,and an audio amplifier 27. The window detector 23 produces a signalproportional to the location of the received signal position within thepass band of the receiver based on the DC component of the output signalof the receiver circuits 21. The output signal of the window circuit issubjected to a threshold test in the conventional receiver. In theinvention, a second threshold test, independent of the conventionalreceiver's threshold test, is applied to determine whether the signalbeing received is near the center of the receiver pass band. The squelchdetector 25 indicates whether a signal is being received by thereceiver. The squelch detector 25 produces a two-state output signal, afirst output that is used as a muting signal to prevent audio noise frombeing heard when no signal is being received. That noise disappears whena signal is actually being received by the receiver. In that event, theother state output signal is supplied by the squelch detector 25 as anun-muting signal that permits the audio in the signal being received tobe reproduced and heard. The un-muting and muting signals are suppliedto and control operation of the audio amplifier 27 so that audio is onlyheard when a signal is being received. When the squelch and windowdetectors indicate that a signal is being received and is within thereceiver pass band, the audio is produced at an audio output device 29.FIG. 1 indicates output through a speaker as a generic audio output, butheadphones and other sound reproducing apparatus can be employed aswell.

The radio receiver A is controlled in its operation by a receivercontrol microprocessor 31. The microprocessor 31 receives the outputs ofthe window detector 23 and squelch detector 25, processes their outputs,and sends an un-muting signal, when appropriate, to the audio amplifier27. The microprocessor 31 controls frequency tuning of the receiver bysending a tuning signal to the tuner within the conventional receivercircuits 21. In coordination with that tuning, the microprocessorcontrols the particular filter element 3-1-3-5 employed in a particularreceiving state in coordination with the tuning of the receiver circuits21. Further, a display 33 is connected to the microprocessor 31 todisplay various information, for example, the channel to which the radioreceiver A is tuned and information used, for example, in programming orcontrolling the apparatus generally. Likewise, a keyboard 35 throughwhich instructions and information are supplied to the apparatus, forexample, in programming and providing other instructions for operationof the radio receiver A, is connected to the microprocessor 31. Finally,the microprocessor 31 includes a data port 37 receiving a signal fromthe microcontroller 11, indicating a frequency determined by themicrocontroller and used in tuning the radio receiver A as describedbelow. Likewise, the connection provides for sending of control signalsfrom the microprocessor 31, for example, regarding a frequency to whichthe receiver is tuned, to the microcontroller 11. That information mightbe used in establishing a divisor of a second prescaler withinmicrocontroller 11 that is described below. The conventional receivercircuits 21 include an oscillator generating a highly precise clock forsynthesized tuning of the receiver A. That clock is preferably also usedby the microcontroller 11 so that a second precision oscillator, arelatively expensive circuit, is unnecessary. The sharing of a commonhigh precision clock circuit between the radio receiver A and thefrequency determining apparatus B is another example of an economyachieved in an embodiment in which the receiver and frequencydetermining apparatus are commonly housed.

Radio Receiver Operation

The operation of the radio receiver A and of the frequency determiningapparatus B is described in conjunction with the flow charts of FIGS. 2and 3. Turning first to the flow chart of FIG. 2, in step S1, if thereis a multiple element bandpass filter, one of the filter elements isselected, i.e., a normally open switch is closed, thereby connecting theselected filter element to the preamplifier 5. The amplified radiofrequency signal is also supplied, for the specific embodiment depictedin FIG. 1, to the preamplifier 7, if present, and, thereafter, asindicated in step S2, to the prescaler 9, if present. For the purposesof this example, it will be assumed that the first prescaler 9, ifpresent, has a fixed value first divisor. That first divisor of thefirst prescaler 9 is applied to produce an output signal of reducedfrequency as compared to the input signal in Step S2. In step S3, thefrequency of the signal supplied to the microcontroller 11 isdetermined. That process in the microcontroller 11 of frequencydetermination apparatus is described below in connection with and aspart of the description of FIG. 3.

In one embodiment of the specific application being described, thefrequency determined in step S3 is employed to tune the radio receiver Ato a channel that incorporates the frequency but may have a centerfrequency different from the frequency determined in step S3 by themicrocontroller 11. The tuning of the radio receiver A is achieved bythe receiver control microprocessor 31 in response to a signalindicating the determined frequency and supplied to the data port 37 ofthe receiver control microprocessor 31. Once that tuning has beenaccomplished, the microprocessor 31 responds to the outputs of one orboth of the window and squelch detectors 23 and 25. One or both of theseresponses may be tested. Preferably, the failure of either test,indicating improper tuning, terminates the frequency determination cycleunderway and the process returns to step S1.

The output of the squelch detector may be tested to determine if it is amuting signal or an un-muting signal. If the squelch circuit is quietedby an incoming signal and produces the un-muting signal, there isconfirmation that a signal is present in the receiver pass band. If theoutput of the window detector indicates that a signal being received isnear the center of the pass band of the receiver, then there is initialor further confirmation that the channel corresponding to the frequencyof the received signal has been accurately determined. If any ofwhichever of these optional window detector and squelch detector testsis applied fails, then microprocessor 31 recognizes that no signal isbeing received and the radio receiver A may revert to its former tuning.The microprocessor 31 includes a memory and, upon successfulverification that a received signal has been accurately tuned, applyingone, both, or neither of the window detector and the squelch detectortests, the channel of the received signal may be stored in step S7 inthe memory as frequency information for future reference in monitoringtransmission of the channel.

An important feature of the invention concerns the tuning function. Thefrequency determination process operates in the background while theradio receiver A operates under other control instructions from themicroprocessor 31, for example, fixed in tuning to a channel where atransmission is present or scanning for transmissions on establishedchannels of interest. In a preferred embodiment, once a preliminaryfrequency determination has been made in step S3 and output to thereceiver controller in step S4, the receiver is tested in test S5 todetermine whether the receiver is currently receiving a signal. Thistest S5 is made by checking whether the squelch detector is producing amuting or un-muting signal. If an un-muting signal is being produced bythe squelch detector indicating that a signal is being received, theprocess moves to step S6 where the new frequency determination isdiscarded. Then the process returns to step S1.

If, in test S5, it is determined that the squelch detector is producinga muting signal so that no signal is currently being received, then theprocess moves to step S7 and the receiver is tuned to the channelincluding the newly determined frequency. Thereafter, the signal on thatchannel is subjected to one or both of the squelch detector and windowdetector tests in test S8. Most preferably, at least the squelchdetector test is applied. Then, upon passage of whatever tests areapplied at test S8, the receiver may remain retuned to monitor thechannel including the newly determined frequency and the channelincluding the newly determined frequency may be stored in a memory inthe receiver in step S9. Alternatively, the receiver may resume scanningfrequencies after storing the frequency newly determined. If any of thetest or tests applied at test S8 are not passed, the process returns tostep S1.

The memory within the receiver control microprocessor 31 may include alist of frequencies that are within or outside the pass band of thefilter 3 but are not to be monitored. Transmissions of paging systemsprovide one example. Then, before any automatic retuning of the receiverto a channel including a frequency determined or storage of acorresponding channel, a comparison is made between the list of excludedfrequencies and the determined frequency. If the frequency determined orits corresponding channel is in the exclusionary list, then thedetermined frequency is discarded and the receiver is not retuned formonitoring or further testing and verification of the frequencydetermined and no new channel information is stored.

The radio receiver and frequency determining apparatus described areparticularly useful in monitoring communications of nearby mobiletransmitters. The nearby mobile transmitters produce relatively strongsignals that may be of particular interest to listeners because thecommunications concern the area near the listener. With the invention,these relatively strong signals can be detected and their frequenciesdetermined. In some communications systems, mobile transmitters do nottransmit and receive directly from a base station but, rather, usehigher power repeater stations as an intermediary. Thus, the low powermobile transceiver effectively covers a much larger geographical area ofcommunication than its transmitter would otherwise provide. In manyfrequency bands, the rules of the Federal Communications Commissionrequire a fixed offset relationship between the frequencies used by themobile units and the repeater frequencies which retransmit transmissionsfrom the mobile units. Optionally, the memory of the receiver controlmicroprocessor 31 retains information on the frequency offsets of thefrequency allocation rules. Then, upon detection of a mobiletransmission, the microprocessor 31 can automatically determine thefrequency at which the corresponding repeater channel transmits and tunethe receiver to that frequency where continued reception of both sidesof the communication, i.e., transmissions of both the remote and nearbyparty, is more likely. After retuning to the repeater channel, one orboth of the window and squelch tests can be applied to ensure that thereceiver is properly tuned to a repeater frequency. If any of theapplied tests fails, then the offset calculation may be inappropriatefor the received signal so that the receiver does not remain tuned tothe calculated frequency. Optionally, the receiver may be retuned to thechannel corresponding to the frequency originally determined.

In the apparatus illustrated in FIG. 1, the switched bandpass filtersare commonly used by the frequency determining apparatus and the radioreceiver. As already described, although two radio frequencypreamplifiers are illustrated in FIG. 1, a single preamplifier may beused for both functions and the radio receiver A and the microcontroller11 can use a single high precision oscillator for certain operations. Infact, although not illustrated, a single microprocessor may be used inplace of the microprocessor 31 and microcontroller 11, performing boththe receiver control and frequency determination functions. These andstill other common circuits may be shared in combining the receiver andthe frequency determining apparatus into an economical single packageunit.

The invention provides an apparatus that is simple, lightweight, andinexpensive, and is, therefore, particularly useful in application withfrequency agile radio receivers. In that usage, the frequencydetermining apparatus may be packaged in a housing separate from afrequency agile radio receiver apparatus. The apparatus may include acable and connector for connection to an input port of such a frequencyagile radio receiver. In that event, the apparatus provides sufficientcommand and frequency information to store a frequency determined in amemory of the frequency scanning radio receiver or to tune the frequencyscanning radio receiver to the channel including the frequencydetermined. More conveniently, as already discussed, the frequencydetermining apparatus is packaged within the same housing as thefrequency agile radio receiver using that circuitry of the receiver thatis common to the frequency determining apparatus as part of thecircuitry of the frequency determining apparatus.

Frequency Determination

The frequency determination by the frequency determination apparatus Bof FIG. 1 is now described with reference to the flow chart of FIG. 3.This apparatus and the associated process are entirely independent ofthe radio receiver apparatus A but described above in conjunction withthe radio receiver to demonstrate one particularly useful application ofthis aspect of the invention. Further, the description that followsassumes the presence of the first prescaler 9, but that optional elementand its associated steps in the flow chart of FIG. 3 may be omitted. Aspreviously noted, the frequency determining apparatus B, in someembodiments and applications, may be no more than the microcontroller 11appropriately connected and programmed. The process described below foraccumulating and manipulating these counts is a preferred, but notexclusive, process since various mathematically equivalent alternativesmay be used to achieve the same end.

The microcontroller 11 includes several registers in which zerocrossings of an input signal are counted and/or accumulated. One of theregisters of the microcontroller 11 is referred to here as a frequencycount register. In the first step S11 of the process of FIG. 3, thatfrequency count register is initialized to zero, i.e., is cleared.

Preferably, the microcontroller 11 includes a prescaling functiondividing the frequency of the input signal to aid in processing of thesignal and determining its frequency accurately and quickly. Thespecific microcontroller identified above includes such a feature,referred to here as a second prescaler, having a second divisor. Thesecond prescaler may have a selectable divisor, for example, any binarynumber between 1 and 256. The second prescaler may also be unused, inwhich case the prescaler divisor is, effectively, 1. In preferredembodiments of the present invention, the second divisor of theprescaler within the microcontroller 11 is 1, 2, or 4, depending uponthe frequency of the signal received

In order to establish the second divisor of the second prescaler withinthe microcontroller 11, in step S12 the microcontroller briefly monitorsthe incoming signal with a second divisor of 1 and counts the zerocrossing transitions of the signal to obtain a prescaling countindicating, in step S13, an apparent frequency of the input signal. Thetime period for monitoring the frequency of the signal received needsonly to be sufficiently long to obtain a rough estimate of thefrequency. An exemplary time period for determining the second divisoris 10 microseconds, during which time a sufficient number of zerocrossing, i.e., amplitude transitions, is observed to estimate thefrequency with an accuracy of about 1 to 5 MHz and establish the seconddivisor set in step S14.

In the invention, as readily understood by one of skill in the art, theamplitude of the signal having a frequency to be determined isunimportant. The first prescaler 9, if present, is preferablyinsensitive to the maximum amplitude of the received signal andpreserves only phase information so that frequency can be accuratelydetermined. One cycle of the signal is determined to have occurredbetween each pair of successive same-direction (e.g., positive tonegative sign) zero amplitude transitions. One zero amplitude transitionoccurs at each time the amplitude of the signal changes in sign, eitherfrom positive amplitude to negative amplitude or from negative amplitudeto positive amplitude. Either these same-direction zero amplitudetransitions or the successive different direction zero amplitudetransitions are observed and counted in order to determine the frequencyof the incoming signal, i.e., the number of effective cycles counted inone second. Since the “count” resulting from the counting of onlysame-direction transitions and from counting all transitions differs bya factor of two (ignoring a potential quantization error discussedelsewhere), it is not important which count is used so long as afrequency calculation from a count properly considers the factor. Thepreferred microcontroller counts only same-direction transitions. Unlessexpressly specified otherwise here, “counting zero amplitudetransitions” and similar terms encompass both alternatives of countingonly same-direction amplitude transitions and counting all amplitudetransitions.

The counting of zero amplitude transitions begins at step S15 where thecounted amplitude transitions increment a count maintained in thefrequency count register that was set to zero in step S11. In thespecific microcontroller identified above as usable in the frequencydetermining apparatus, a free-running counter TMR0 is incremented inresponse to each cycle at the output of the prescaler 9. Thisfree-running counter is the lowest byte of the frequency count register.The running count of amplitude transitions is accumulated in step S17 ofan initial part of the frequency determination process.

In one application of the invention, as already described, the frequencydetermining apparatus B is used to tune the frequency of a frequencyagile radio receiver. The tuning is made to particular channels ratherthan to specific frequencies. The channel spacing, i.e., a frequencyallocation, depends upon the transmitting frequency. For example, athigher frequencies, such as 900 MHz, channel spacing may be 12.5 kHz. Atlower frequencies, the channel spacing may be only 5 kHz. Therefore, inorder to determine accurately the highest frequency of the frequenciesof usual interest, a precision of about 1 part in 72,000 (i.e.,900×10⁶÷12.5×10³) is required. In digital signal terms, at least 72,000transitions must be counted in order to determine the highest frequencyof usual interest with the precision necessary for this application ofthe invention.

In typical digital signal processing, 8 bit registers, for which themaximum count is 255, are employed. Therefore, the basic 8 bit register,e.g., the lowest byte free-running counter TMR0 referred to above, inthe microcontroller 11 that responds to each transition from theprescaler 9, if present, or to the radio frequency signal directly ifthe prescaler 9 is omitted, is insufficient in maximum counting capacityfor the receiver application described here. To provide the necessarycount capacity for a precision of one part in 72,000, several 8 bitregisters may be used in combination. The use of multiple registers canbe accomplished in various ways known to those of skill in the art. Forexample, registers may be conceptually assembled end-to-end, throughprogramming, so that each overflow bit from a first register is countedin a second 8 bit register, and overflow bits from the second registerare counted in a third register. Alternatively, a register may bemonitored at regular intervals to determine when each overflow occursand another register incremented for each such detected overflow.

An important feature of the frequency determination aspect of theinvention is the concept of coherency and the determination of thefrequency only of “coherent” signals. Incoherent signals are discardedin the course of the frequency determination according to the invention.This feature of the invention is particularly important in applicationsof the invention to radio receivers so that signals on differentfrequencies are not confused. Likewise, by determining the frequency ofonly coherent signals, random noise is rejected in the invention. Thecoherency test is achieved in the invention by establishing a baselinecount of zero amplitude transitions, i.e., zero crossings, for aninitial time period of a received signal, extracting a plurality ofequal duration time periods from the remaining part of a sampling time,counting zero amplitude transitions in each of those equal duration timeperiods, successively, and, if there is disagreement between thebaseline count and a count in a subsequent time period, determining thatthe signal is not coherent. Upon determination of lack of coherencyduring or after counting, the partial or complete results of thefrequency determining cycle are discarded. In other words, either afrequency determination in progress is terminated or the result of acomplete frequency determination is discarded. A specific example ofsuch a process is now described with reference to FIG. 3.

More specifically, the zero amplitude transitions of the received signalare counted by incrementing the frequency count register in step S16 fora first time period of fixed duration. The first and subsequent timeperiods of equal duration are sometimes referred to here as time slices.Whether the end of this first time slice has been reached is tested intest S17 after each increment of the frequency count in step S16. Anexemplary, but non-limiting, example of the duration of such a timeslice is 8 milliseconds. When the test S17 is fulfilled, the value ofthe count of transitions for the first such time slice is established asthe base line count for this sampling. Meanwhile, the counting ofamplitude transitions continues, with the frequency count register countcopied into a previous count register at step S19. The previous countregister is used in further steps of the process.

In the second and subsequent time slices identical in duration to thefirst time slice, amplitude transitions are counted at test S20 as intest S15, with each count incrementing the frequency count register atstep S21. The end of each time slice is detected in test S22. When theend of each subsequent time slice is identified at test S22, adetermination is made in step S23 as to whether the received signal hasremained coherent. If a received signal is coherent, then the slicecount during each of the time slices of equal duration will be exactlythe same subject only to an identifiable error. This error, typicallyreferred to in the art as “quantization error”, may occur because of thephase of the incoming signal at the beginning and the end of each timeslice. The error cannot exceed plus or minus 1 count.

Numerous mathematically equivalent tests may be applied to determinecoherency, i.e., whether counts for each time slice do not vary by morethan the quantization error. In the preferred embodiment, illustrated inFIG. 3, in step S23, the count stored in the frequency count register,i.e., the total accumulated count at that time, has subtracted from itthe count in the previous count register, a count established at stepS19. The resulting difference is the count for the time slice justconcluded. That difference is stored in a register referred to here as acoherence count register as step S23. In the test S24, the differencestored in the coherence count register is compared to the count in thebaseline register, i.e., the count from the first time slice. If thecount in the coherence count register differs from the count in thebaseline register by more than 1 count, then the received signal is notcoherent, the frequency determination in process is aborted, and theprocess returns to step S11, resetting the frequency count register tozero and starting the frequency determination process again

For each subsequent time slice for which there is agreement, withinquantization error, of the count of the coherence count register and thecoherence baseline register, the process moves from test S24 to test S25as the frequency count register accumulates a total count. Theaccumulation continues until the end of a total count time without afailure at test S24. For example, in the non-limiting radio receiverembodiment previously mentioned, sixteen such time slices of 8milliseconds duration each, making for a total count of 256milliseconds, are used. The total count time is entirely arbitrary as isthe duration of each “slice”. It is convenient, but not required, thatthe total count time be an integer multiple of the duration of each“slice” period. In any event, when the total count time ends, theprocess passes from test S25 to step S26 where frequency is calculatedfrom the total count accumulated in the frequency count register.

Alternatively, the comparison of the counts for each of the time slices,i.e., periods, to the count for the first time period may be delayeduntil after the end of the total count time. In that alternativeembodiment, with reference to FIG. 3, test S24 is postponed until aftertest S25. Then test S24 is carried out, before the frequency calculationat step S26. Since, in this alternative, no comparison of time slicecounts is made until the end of the total count time, there is notermination of the frequency determination process before the end of thetotal count time, as in the embodiment previously described.

Still more generally, since the objective of the comparison of countsfor each time slice is to determine coherency, i.e., whether the countsvary by more than the quantization error, the count for any time slicecan be used as the reference count. In other words, one arbitrarilychosen time slice count could be selected, particularly after all timeslice counts have been obtained at the end of the total count time, andthat selected time slice count compared to all other time slice counts.A time slice count for a time slice other than the first time slice maybe selected at any time during passage of the total count time. Then,the selected reference time slice count is compared to the other timeslice counts to determine whether there is agreement within thequantization error. With regard to FIG. 3, in this alternative, the stepS18 is postponed and occurs at a particular time slice n, where n is aninteger. In any event, for best efficiency in processing time and tominimize memory requirements, it is preferred to use the first timeslice count as the reference count. Nevertheless, the describedalternatives provide equivalent conceptual and practical results.

The accumulated count in the frequency count register is an indicator ofand is proportional to the frequency of the signal received. In order toobtain the actual frequency of the signal processed, it is necessary tomultiply the total count in the frequency count register by the firstand second divisors of the first and second prescalers, if present andused, and to divide by the total count time. For example, in theexemplary embodiment described, the total count is multiplied by 512when the first prescaler 9 divides the frequency by 128 and the secondprescaler divides the frequency by 4, and is divided by 0.256 when thetotal count time is 256 ms. The frequency determined is an output signalof the microcontroller 11.

The performance of the frequency determining apparatus of FIG. 1 issubstantially improved over prior art frequency measurements. A singlesampling of a radio frequency signal of sufficient duration to receive acoherent signal is used to determine frequency, providing a more validand sure response than prior art apparatus requiring multiple samples todetermine a potentially inaccurate frequency from one or more incoherentsamples. Moreover, unlike the prior art apparatus, in the invention thecounting for determining frequency begins at the earliest coherentreception. This feature provides rapid response, for example,determination of frequency and retuning an associated frequency agileradio, typically, in a fraction of a second. Fading signals are morereliably detected and monitored with the invention than in prior artapparatus.

Transceiver Embodiment

In another aspect of the invention, the radio receiver embodiment ofFIG. 1 can be modified to function as a transceiver, as shown in FIG. 4.The radio receiver apparatus A is replaced by a radio transceiverapparatus C including all of the parts of the radio receiver A plus atransmitter 41 connected to the antenna 1 through a transmit/receiveswitch 43. The transmitter 41 is conventional and is frequency agile,with its tuning being controlled with the same tuning signal supplied bythe microprocessor 31 to the receiver circuits 21. When the transmitter41 is present, upon determination of the frequency of a nearbytransmission, both the receiver circuits 21 and the transmitter 41 maybe automatically tuned to a channel including the frequency orfrequencies (in the event of diplex communication) of communication. Inthat event, the user of the transceiver can immediately enter into aconversation that is already underway. This feature can be of particularvalue to EMS, fire, and other rescue personnel.

The invention has been described with respect to certain preferredembodiments. Various additions and modifications within the spirit ofthe invention will occur to those of skill in the art from the foregoingdescription. The invention, as defined by the following claims,encompasses all such modifications and variations.

1. A radio apparatus comprising: a frequency agile radio receiverincluding an antenna for collecting electromagnetic energy, a bandpassfilter connected to the antenna and having a pass band passing radiofrequencies within a first range of radio frequencies, and rejectingfrequencies outside the first range of frequencies; receiver circuitsfor processing radio frequency signals received through the antenna andproducing an output, a radio frequency preamplifier amplifying a radiofrequency signal received from the antenna through the bandpass filter;and a microprocessor controlling tuning of the frequency agile radioreceiver; and a frequency determining apparatus for determining thefrequency of a radio frequency signal received from the antenna andthrough the bandpass filter and supplying the frequency determined tothe microprocessor, wherein the microprocessor tunes the frequency agileradio receiver to the frequency determined, wherein the frequencydetermining apparatus includes a counter for counting zero amplitudetransitions of a radio frequency signal in each of a plurality of timeperiods of identical duration; a first register for storing counted zeroamplitude transitions for a selected one of the time periods; a secondregister for storing an accumulated count of zero amplitude transitionscounted during a total counting time including the plurality of timeperiods; and comparing means for comparing counted zero amplitudetransitions for each time period other than the selected time periodwith the counted zero amplitude transitions for the selected timeperiod, the second register discarding the accumulated count if thecounted zero amplitude transitions for any of the time periods otherthan the selected time period are different from the counted zeroamplitude transitions for the selected time period by more than aquantization error, wherein the frequency of the radio frequency signalis indicated by the accumulated count stored in the second register whenthe total counting time ends.
 2. The radio apparatus according to claim1 wherein the frequency agile radio receiver includes a squelch detectorreceiving the output of the receiver circuits and, when a radiofrequency signal received causes the receiver circuits to produce theoutput, generating an un-muting signal for output of sound by the radioreceiver, the microprocessor controls the frequency agile radioreceiver, in response to a frequency determination by the frequencydetermining apparatus, to tune to a channel including the frequencydetermined, and the frequency agile radio receiver determines whetherthe squelch detector produces the un-muting signal, and, if not,disregards the frequency determined.
 3. The radio apparatus according toclaim 1 wherein the frequency agile radio receiver has a pass band andincludes a window detector receiving the output of the receiver circuitsand indicating whether a received radio frequency signal is centeredwithin the pass band of the frequency agile radio receiver, themicroprocessor controls the frequency agile radio receiver, in responseto a frequency determination by the frequency determining apparatus, totune to a channel including the frequency determined, and the frequencyagile radio receiver determines whether the window detector indicatesthat the radio frequency signal is centered within the pass band of thereceiver, and, if not, disregards the frequency determined.
 4. The radioapparatus according to claim 1 wherein the frequency agile radioreceiver includes a memory for storing frequency information in which achannel corresponding to a frequency determined by the frequencydetermining apparatus is stored.
 5. The radio apparatus according toclaim 1 wherein the frequency agile radio receiver includes a memorycoupled to the tuner for storing frequency information for channelsexcluded from reception by the radio receiver, and when the frequencydetermining apparatus determines the frequency of a radio frequencysignal that falls within a channel excluded from reception, thefrequency is discarded and the frequency agile radio receiver is nottuned to the channel corresponding to the frequency determined.
 6. Theradio apparatus according to claim 1 including a demodulator wherein,after the microprocessor tunes the frequency agile radio receiver to thefrequency determined, the demodulator produces an audio signal from aradio frequency signal received at the frequency determined.
 7. A radioapparatus comprising: a frequency agile radio receiver including anantenna for collecting electromagnetic energy, a bandpass filterconnected to the antenna and having a pass band passing radiofrequencies within a first range of radio frequencies, and rejectingfrequencies outside the first range of frequencies; receiver circuitsfor processing radio frequency signals received through the antenna andproducing an output, a radio frequency preamplifier amplifying a radiofrequency signal received from the antenna through the bandpass filter;and a microprocessor controlling tuning of the frequency agile radioreceiver; and a frequency determining apparatus for determining thefrequency of a radio frequency signal received from the antenna andthrough the bandpass filter and supplying the frequency determined tothe microprocessor, wherein the microprocessor tunes the frequency agileradio receiver to the frequency determined, wherein the frequencydetermining apparatus includes a counter for counting zero amplitudetransitions of a radio frequency signal in each of a plurality of timeperiods; a register for storing counted zero amplitude transitions for aselected one of the time periods; and comparing means for comparingcounted zero amplitude transitions for a time period other than theselected time period with the counted zero amplitude transitions for theselected time period, and, if the counted zero amplitude transitions fora time period other than the selected time period are different by lessthan a threshold value, tuning the frequency agile radio receiver to thefrequency determined.
 8. The radio apparatus according to claim 7wherein the frequency agile radio receiver includes a squelch detectorreceiving the output of the receiver circuits and, when a radiofrequency signal received causes the receiver circuits to produce theoutput, generating an un-muting signal for output of sound by the radioreceiver, the microprocessor controls the frequency agile radioreceiver, in response to a frequency determination by the frequencydetermining apparatus, to tune to a channel including the frequencydetermined, and the frequency agile radio receiver determines whetherthe squelch detector produces the un-muting signal, and, if not,disregards the frequency determined.
 9. The radio apparatus according toclaim 7 wherein the frequency agile radio receiver has a pass band andincludes a window detector receiving the output of the receiver circuitsand indicating whether a received radio frequency signal is centeredwithin the pass band of the frequency agile radio receiver, themicroprocessor controls the frequency agile radio receiver, in responseto a frequency determination by the frequency determining apparatus, totune to a channel including the frequency determined, and the frequencyagile radio receiver determines whether the window detector indicatesthat the radio frequency signal is centered within the pass band of thereceiver, and, if not, disregards the frequency determined.
 10. Theradio apparatus according to claim 7 wherein the frequency agile radioreceiver includes a memory for storing frequency information in which achannel corresponding to a frequency determined by the frequencydetermining apparatus is stored.
 11. The radio apparatus according toclaim 7 wherein the frequency agile radio receiver includes a memorycoupled to the tuner for storing frequency information for channelsexcluded from reception by the radio receiver, and when the frequencydetermining apparatus determines the frequency of a radio frequencysignal that falls within a channel excluded from reception, thefrequency is discarded and the frequency agile radio receiver is nottuned to the channel corresponding to the frequency determined.
 12. Theradio apparatus according to claim 7 including a demodulator wherein,after the microprocessor tunes the frequency agile radio receiver to thefrequency determined, the demodulator produces an audio signal from aradio frequency signal received at the frequency determined.
 13. A radioapparatus comprising: a frequency agile radio receiver including anantenna for collecting electromagnetic energy, a bandpass filterconnected to the antenna and having a pass band passing radiofrequencies within a first range of radio frequencies, and rejectingfrequencies outside the first range of frequencies; receiver circuitsfor processing radio frequency signals received through the antenna andproducing an output, a radio frequency preamplifier amplifying a radiofrequency signal received from the antenna through the bandpass filter;a first memory for storing frequency information for tuning of thefrequency agile radio receiver; and a microprocessor controlling tuningof the frequency agile radio receiver; and a frequency determiningapparatus for determining the frequency of a radio frequency signalreceived from the antenna and through the bandpass filter and supplyingthe frequency determined to the first memory for storage for access bythe microprocessor for tuning the frequency agile radio receiver,wherein the frequency determining apparatus includes a counter forcounting zero amplitude transitions of a radio frequency signal in eachof a plurality of time periods; a register for storing counted zeroamplitude transitions for a selected one of the time periods; andcomparing means for comparing counted zero amplitude transitions for atime period other than the selected time period with the counted zeroamplitude transitions for the selected time period, and, if the countedzero amplitude transitions for a time period other than the selectedtime period are different by less than a threshold value, storing thefrequency in the memory for access by the microprocessor for tuning thefrequency agile receiver.
 14. The radio apparatus according to claim 13wherein the frequency agile radio receiver includes a squelch detectorreceiving the output of the receiver circuits and, only when a radiofrequency signal received causes the receiver circuits to produce theoutput, generating an un-muting signal for output of sound by the radioreceiver, the microprocessor, in response to a frequency determinationby the frequency determining apparatus, controls the frequency agileradio receiver to tune to a channel including the frequency determined,and the frequency agile radio receiver determines whether the squelchdetector produces the un-muting signal, and, if not, disregards thefrequency determined.
 15. The radio apparatus according to claim 13wherein the frequency agile radio receiver has a pass band and includesa window detector receiving the output of the receiver circuits andindicating whether a received radio frequency signal is centered withinthe pass band of the radio receiver, the microprocessor controls thefrequency agile radio receiver, in response to a frequency determinationby the frequency determining apparatus, to tune to a channel includingthe frequency determined, and the frequency agile radio receiverdetermines whether the window detector indicates that the radiofrequency signal is centered within the pass band of the receiver, and,if not, disregards the frequency determined.
 16. The radio apparatusaccording to claim 13 wherein the frequency agile radio receiverincludes a second memory coupled to the tuner for storing frequencyinformation for channels excluded from reception by the radio receiver,and when the frequency determining apparatus determines the frequency ofa radio frequency signal that falls within a channel excluded fromreception, the frequency is discarded and the frequency agile radioreceiver is not tuned to the channel corresponding to the frequencydetermined.
 17. The radio apparatus according to claim 13 wherein thefrequency agile radio receiver includes a squelch detector receiving theoutput of the receiver circuits and, only when a radio frequency signalreceived causes the receiver circuits to produce the output, generatingan un-muting signal for output of sound by the radio receiver, and themicroprocessor controls the frequency agile radio receiver to tunesequentially to frequencies included in the frequency information storedin the memory, and when the squelch detector produces the un-mutingsignal for any of the frequencies to which the frequency agile radioreceiver is tuned, controls the frequency agile radio receiver to remaintuned to the frequency, and when the squelch detector does not producethe un-muting signal, controls the frequency agile radio receiver totune to another of the frequencies.